Optical sensor having heating element to heat amorphous semiconductor film

ABSTRACT

In an optical sensor, a heating element is formed on a substrate, and an amorphous semiconductor film is formed on an insulating layer covering the heating element, and is electrically insulated from the heating element. A common electrode and a plurality of electrodes are also formed on the substrate and are extended along the amorphous semiconductor film, to form cells for converting light into electrical signals, in the amorphous semiconductor film. An electric current is supplied to the heating element, to heat the amorphous semiconductor film after the film has been illuminated and photoelectric current has been picked up from the electrodes.

BACKGROUND OF THE INVENTION

The present invention relates to an optical sensor which can be used as,for example, an image sensor, and more particularly to an optical sensorprovided with an amorphous film, functioning as a photoconductive film.

Amorphous semiconductor film, a typical example of which is amorphoussilicon film, can be deposited on a substrate by means of glow dischargeof a gas, such as SiH₄, or a combination of SiH₄ with H₂, PH₃, B₂ H₆and/or CH₄. Further, it can be formed on a large surface area of thesubstrate. Due to these advantageous features, amorphous semiconductorfilm has attracted much attention in the art, since it can be used as aphotoelectric conversion film in image sensors.

One of the various known optical sensors using amorphous semiconductorfilms has an electrode placed in ohmic contact with the amorphoussemiconductor film. When the film is illuminated, it undergoesphotoconduction, and its resistance changes. As a result, the filmgenerates a photo-current, which is supplied as a signal from theelectrode. The semiconductor amorphous film and the signal electrodeform a photoelectric transducer element, usually called a "cell", of aphotosensor. FIG. 1 is a graph illustrating the relationship between thevoltage applied to the cell and the photoelectric current generated bythe cell, as the amount of light irradiated to the cell is varied. Asthis figure shows, the greater the amount of light, the higher theresistance of the amorphous semiconductor film. FIG. 2 illustrates therelationship between the luminance on the cell surface and thephotoelectric current generated by the cell. As is evident from FIG. 2,the photoelectric current increases as the luminance increases. Hence, apredetermined electric current can be obtained by applying anappropriate voltage to the cell.

It has been ascertained that the longer an amorphous semiconductor filmis exposed to light, the higher the resistance the film will have, andhence, the smaller the photoelectric current will become. Thisphenomenon is known in the as Staebler and Wronski effect. That is, thephoto-current gradually decreases as the film is continuouslyilluminated, even if the luminance (E) remains unchanged, as can beunderstood from the graph of FIG. 3. The decrease of the photoelectriccurrent, due to this effect, is prominent in proportion to the luminance(E). The amorphous semiconductor film has its photosensitivity sharplyreduced in a relatively short time. Therefore, the film cannot bepractically used in an optical sensor.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an optical sensorwhich can compensate for the Staebler and Wronski effect in an amorphoussemiconductor film, and which can, therefore, remain highlyphotosensitive for a long period of time.

According to the present invention, there is provided an optical sensorwhich comprises a substrate, a heating element formed on the substrate,an insulating layer formed on the heating element, and an amorphoussemiconductor film (a photoelectric conversion film) formed on theinsulating layer. An electric current is supplied to the heatingelement, at a predetermined interval, upon lapse of predetermined timeafter light has been applied to the amorphous semiconductor film, orevery time the photoelectric current, generated by the light applied tothe film, decreases below a predetermined value. Whenever the heatingelement is supplied with an electric current, it heats the amorphoussemiconductor film, whereby the film deteriorated by the application oflight, regains its photoelectric conversion characteristic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between the voltage appliedto an optical sensor having an amorphous semiconductor film used as aphotoconductive film, and the photoelectric current generated by theoptical sensor, when different amounts of light are applied to theoptical sensor;

FIG. 2 is a graph representing the relationship between the luminance onthe optical sensor and the photoelectric current generated by thesensor, as different voltages are applied to the optical sensor;

FIG. 3 is a graph illustrating the relationship between the time ofapplying light to the optical sensor and the photoelectric currentgenerated by the sensor, as the amount of light applied to the sensor ischanged;

FIG. 4A is a plane view of an image sensor according to one embodimentof the invention;

FIG. 4B is a cross-sectional view of the image sensor, taken along lineB--B in FIG. 4A;

FIG. 5 is a graph representing the relationship between the temperatureto which the amorphous semiconductor film of an optical sensor has beenheated, and the time which the deteriorated film needs, in order toregain its photoelectric conversion characteristic;

FIG. 6 is a graph showing the relationship between the time of supplyinga current to the heating element of the image sensor shown in FIGS. 4Aand 4B, and the temperature of the heating element;

FIG. 7 is a cross-sectional view of an image sensor according to anotherembodiment of the present invention.

FIG. 8A is a plane view of an image sensor according to anotherembodiment of the invention; and

FIG. 8B is a cross-sectional view of the image sensor, taken along lineC--C in FIG. 8A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 4A and 4B show a one-dimensional image sensor according to oneembodiment of this invention. This image sensor comprises insulativesubstrate 1 made of glass or ceramic, or comprised of a ceramic plateand a glass layer formed on one surface of the ceramic plate. The sensorfurther comprises strip-like heating element 2 formed on substrate 1,insulating layer 4 covering heating element 2, and amorphoussemiconductor film 5 formed on insulating layer 4 and hence,electrically insulated from heating element 2. Heating element 2 iseither a thick resistive member or a thin resistive member of TaSiO,BaRuO₃, Poli-Si, Cr-SiO₂, Ni-Cr, Ti, W, or Cr, and is electricallyconnected at one end to currentsupplying electrode 3A and at the otherend to currentsupplying electrode 3B. Insulating layer 4 is made of Ta₂O₅, SiO₂, or Si. Amorphous semiconductor film 5 is made of hydrogenatedamorphous silicon (a-Si:H) containing 20% or more of silicon and 10% ormore of hydrogen. It is formed by glow-discharging SiH₄, or acombination of SiH₄ with H₂, PH₃, B₂ H₆ and/or CH₄ gases, therebydepositing amorphous silicon on insulating layer 4. Common electrode 8is formed on substrate 1, in the form of a comb. Teeth 8A ofinterdigitated electrode 8 are juxtaposed, each extending in the widthdirection of amorphous silicon film 5. A plurality of electrodes 7 areformed on substrate 1. Each of electrodes 7 is also shaped into aninterdigitated form and has two teeth 7A. Each of teeth 7A and 8A, andamorphous silicon film 5 form a cell for converting light into anelectrical signal. Electrodes 7 and common electrode 8 are made of ametal such as aluminum, titanium, or manganese. A dopant such asdi-boran can be doped into those portions of amorphous silicon film 5which contact teeth 7A and 8A, thereby to form n⁺ layers 9. When n⁺layers 9 are formed in amorphous silicon film 5, the linearity of thevoltage-current characteristic (FIG. 2) can be improved. To read animage, electrodes 7 are selectively scanned, and each cell generates aphotoelectric current.

As has been explained with reference to FIG. 3, the longer an amorphoussemiconductor film is illuminated, the more its photoelectric conversioncharacteristic will be deteriorated, and the less photoelectric currentit will generate. However, the deteriorated amorphous semiconductor filmcan regain its photoelectric conversion characteristic when it isheated. The time the film needs to regain its photoelectric conversioncharacteristic depends on the temperature to which it has been heated,as is illustrated in the graph of FIG. 5. More specifically, as is shownin FIG. 5, the higher the temperature, the shorter the time the filmrequires to regain its photoelectric conversion characteristic.

The present invention makes full use of the abovementioned nature of anamorphous semiconductor film. As is shown in FIG. 4A, heating element 2is formed on substrate 1, in order to heat amorphous semiconductor film5 formed on insulating layer 4, which in turn is formed on heatingelement 2. Heating element 2 is a heating resistive member of the typeused in a thermal printing head. Its temperature therefore quicklychanges, as is indicated in FIG. 6; it can rise quickly to 150° C. ormore, a temperature high enough to make amorphous semiconductor film 5regain its photoelectric conversion characteristic. An electric currentis supplied to heating element 2, via electrodes 3A and 3B, after film 5has been illuminated and a photo-current has been supplied from eachcell. Element 2 thus generates heat, thereby heating amorphoussemiconductor film 5. Hence, the deteriorated photoelectric conversioncharacteristic of film 5 can be quickly compensated for every time thecells supply the photoelectric current. This ensures highly reliableimage-sensing.

Amorphous semiconductor film 5 need not be heated as frequently as this;it can instead be heated at regular intervals. Alternatively, anelectric current can be supplied to heating element 2 only when theoutput photoelectric current of a reference cell, other than those cellsfor sensing an image, decreases below a predetermined value. Any of theabove methods of heating film 5 can compensate for the deterioratedphotoelectric conversion characteristic of film 5, without sacrificingthe image-sensing speed of the image sensor.

The present invention is not limited to the embodiment described above.For example, as is shown in FIG. 7, heating element 2 can be formed onthe surface of insulative substrate 1, opposite to amorphoussemiconductor film 5. In this case, substrate 1 functions as a layerwhich electrically insulates element 2 from film 5. Hence, insulatinglayer 4 (FIG. 4) can be dispensed with, thus simplifying the structureof the image sensor.

FIGS. 8A and 8B show an optical sensor according to another embodimentof the invention. The sensor shown in FIGS. 8A and 8B is provided with aplurality of sensing areas 10₁ to 10_(n), each of which has the samestructure and arrangement as that of the sensor shown in FIGS. 4A and4B. In the sensor shown in FIGS. 8A and 8B, one heating element 2 isformed on substrate 1, and extends under sensing areas 10_(l) to 10_(n),and a plurality of adhesion-contact patterns 12 made of ahigh-resistivity material, the same as that of heating element 2, arealso formed on substrate 1. Element 2 and patterns 12 are preferablymade of Ti, W, or Cr, and formed at the same time, in the same process.Line electrodes 14 of Au are formed on contact patterns 12,respectively, and are covered by insulating layer 4 having a pluralityof through-holes 16 through which line electrodes 14 are electricallyconnected to corresponding electrodes 7. An electrode cannot be reliablyadhered to substrate 1 made of glass or ceramic, but Au electrode can bereliably adhered to a Ti, W, or Cr layer. Accordingly, adhesion-contactpatterns 12 are provided on substrate 1.

In the sensor shown in FIGS. 8A and 8B, one of common electrodes 8 andone of electrodes 7 are continuously selected, and a voltage is appliedto the selected electrodes 7, 8 from a driver circuit (not shown) viathe corresponding pattern 12 when a photo-current, that is, aphoto-signal, is picked up from the predetermined sensor cell which isdefined by the selected electrodes 7, 8 and a region of film 5therebetween. An electric current is supplied to heating element 2 viaelectrodes 3A and 3B, after film 5 has been illuminated and thephoto-current has been supplied from each cell.

In the sensor shown in FIGS. 8A and 8B, the sensor can be easilymanufactured, even when heating element 2 is formed on substrate 1,because heating element 2 is made of the same material as that ofcontact patterns 12 and heating element 2, and contact patterns 12 areformed at the same time.

According to the present invention, an optical sensor having highreliability can be realized in a simple structure and into a compactdevice. Therefore, the optical sensor of the invention can be preferablyincorporated in various systems, for example, a facsimile system.

Moreover, various alterations and modifications can be made withoutdeparting from the scope and spirit of this invention.

What is claimed is:
 1. An optical sensor comprising:an insulativesubstrate having a first surface and a second surface opposing the firstsurface; an amorphous semiconductor film formed on the first surface ofthe substrate, for converting light rays into electrical signals; afirst electrode and a second electrode both formed on the amorphoussemiconductor film and separated from each other; and a heating elementformed on either the first or second surface of the substrate, forheating the amorphous semiconductor film, when supplied with anelectrical current.
 2. The optical sensor according to claim 1, furthercomprising an insulating layer for electrically insulating the heatingelement from the amorphous semiconductor film, and wherein said heatingelement is formed on the first surface of the substrate, said insulatinglayer is formed partly on the heating element and partly on thesubstrate, and said amorphous semiconductor film is formed on saidinsulating layer.
 3. The optical sensor according to claim 1, whereinsaid heating element is formed on the second surface of said substrateand opposing said amorphous semiconductor film.
 4. The optical sensoraccording to claim 1, wherein an electrical current is supplied to saidheating element, after light rays have been applied to said amorphoussemiconductor film.
 5. The optical sensor according to claim 1, whereinan electrical current is supplied to said heating element when a signalcurrent, which said amorphous semiconductor film generates upon receiptof light rays, decreases below a predetermined value.
 6. The opticalsensor according to claim 1, wherein said heating element is made ofhigh-resistivity material.
 7. The optical sensor according to claim 1,further comprising:an intermediate layer formed on said substrate; athird electrode formed on said intermediate layer and electricallyconnected to said first electrode; and an insulating layer covering saidthird electrode.
 8. The optical sensor according to claim 7, whereinsaid intermediate layer is made of same material as that of said heatingelement.